Method and device for mixing gases

ABSTRACT

Said device comprises a substantially tubular mixing chamber ( 1 ), a first inlet orifice for a stream ( 4 ) of the first gas at one end ( 2 ) of said chamber, a second inlet orifice ( 6 ) for a stream of the second gas, located downstream of said first orifice in the direction of flow of the stream of the first gas, means ( 10 ) for homogeneously mixing said streams of the first and second gases, said means being located downstream of said second inlet orifice, an outlet orifice ( 3 ) for the mixture, located downstream of said mixing means ( 10 ), at the other end of said chamber, and an orifice ( 11 ) for sampling said mixture of the first and second gases, said orifice being located between said mixing means ( 10 ) and said outlet means ( 3 ) for the mixture, said device furthermore including means ( 5, 8, 9 ) for sending the stream of the first gas into said chamber with a controlled flow rate and for sending the stream of the second gas into said chamber with a controlled flow rate.

[0001] The present invention relates to a method for mixing gases.

[0002] The invention also relates to a device for mixing gases.

[0003] The invention relates in particular to a method and to a devicethat allow one gas to be diluted in another gas in dynamic mode,especially in very low concentrations.

[0004] The invention makes it possible to artificially recreateatmospheres that contain one particular compound with a definedconcentration, such as a pollutant, and one application of theinvention, among others, is in the calibration of gas sensors, thecalibration of gas concentration meters, involving the plotting ofcalibration curves, quantification on the basis of preconcentrationsystems, etc.

[0005] The technical field of the invention may be defined as that ofthe mixing of gases and, more particularly, that of diluting one gas inanother, such as air.

[0006] Gas diluting devices may be put into two categories: firstly,static or closed systems and, secondly, dynamic systems.

[0007] In static or closed systems, the products, in known amount, aregenerally vaporized in a glass flask, also of known volume.

[0008] This type of system gives rise to the problem of a loss ofproduct in the case of certain compounds, this being due to adsorptionon the walls of the system, which is particularly sensitive to lowconcentrations.

[0009] In dynamic systems, use is made of a permeation membrane throughwhich the pollutant gas molecules pass.

[0010] These devices have the drawback of relying on statisticalconsiderations and are generally quite difficult to implement.

[0011] There is therefore a need for a method and a device for mixingone gas in another gas, that does not have the drawbacks, limitations,shortcomings and disadvantages of the devices and methods of the priorart.

[0012] There is also a need for a method and a device for mixing one gasin another gas that makes it possible to obtain a mixture in which therespective proportions of each of the gases are defined very precisely,even with very low concentrations of one of the gases.

[0013] This method and this device must furthermore be simple toimplement, and must operate in a completely reproducible manner, withhigh reliability and great steadiness, whatever the gases involved.

[0014] The aim of the present invention is therefore to provide a methodand a device that satisfy, among others, these needs and that meet theserequirements.

[0015] The aim of the present invention is also to provide a method anda device for mixing gases that solve the problems of the methods anddevices of the prior art, whether these be methods and devices of thestatic type or methods and devices of the dynamic type.

[0016] This aim and also others are achieved, in accordance with theinvention, by a method of mixing, in dynamic mode, a second gas in afirst gas, in which a stream of the second gas is introduced into astream of the first gas, said streams of the first and second gaseshaving controlled flow rates, and said streams of the first and secondgases are mixed so as to obtain a homogeneous mixture of the two gasesthat has a defined concentration of the second gas.

[0017] Advantageously, said first gas is chosen from air, nitrogen,argon, helium and mixtures thereof. The preferred gas is air.

[0018] Advantageously, said second gas results from the vaporization ofa liquid compound (under standard temperature and pressure conditions),preferably a compound selected from liquid organic compounds andmixtures thereof.

[0019] Advantageously, said second gas is a compound selected fromcompounds polluting the atmospheric air and mixtures thereof. Thiscompound may, under standard conditions, be in the liquid state or inthe gaseous state.

[0020] These polluting compounds are generally selected from volatileorganic compounds, for example alcohols such as n-butanol or the like.

[0021] Advantageously, according to the invention, a fraction of thehomogeneous mixture of the two gases is sampled and sent into a meterand/or detector and/or concentration meter.

[0022] The method according to the invention makes it possible toartificially recreate a homogeneous gas mixture such as, for example, apolluted atmosphere, in dynamic mode.

[0023] The method according to the invention and the device, which alsoforms the subject matter of the present invention, because they operatein dynamic mode, fundamentally offer the advantages associated with themethods and instruments operating according to that principle, namely,above all, the fact that the problems associated with product beingdeposited on the walls are limited. However, the method and the deviceaccording to the invention, although they offer all the advantages ofdynamic systems, do not, however, have their drawbacks. This is becausethey do not rely on statistical considerations and, especially becauseof the absence of a permeation membrane, they are simple, reliable andeasy to implement, and ensure that homogeneous mixtures are preparedwith precise concentrations, reproducibly and with great stability.

[0024] According to the invention, the flow rate of the first gas, suchas air, and of the second gas are both controlled. It is thus very easyto obtain, with very great reliability, a homogeneous gas mixture, andeven to do so for very low concentrations of the second gas in the firstgas. Knowing the two flow rates, of the first and second gases,extremely precisely makes it possible to calculate, with greatprecision, the theoretical concentration of the second gas in the firstgas, for example the concentration of the pollutant or pollutants in theair. It is possible, according to the invention, to regulate or adjustthe flow rate of each of the gases with great precision, so as to obtainhomogeneous mixtures having all possible concentrations and, inparticular, the concentration range of the desired field of application.These concentrations are each time obtained with great precision in thefinal homogeneous mixture.

[0025] The invention also relates to a device for mixing, in dynamicmode, a second gas in a first gas, said device comprising asubstantially tubular mixing chamber, a first inlet orifice for a streamof the first gas at one end of said chamber, a second inlet orifice fora stream of the second gas, located downstream of said first orifice inthe direction of flow of the stream of the first gas, means forhomogeneously mixing said streams of the first and second gases, saidmeans being located downstream of said second inlet orifice, an outletorifice for the mixture, located downstream of said mixing means, at theother end of said chamber, and an orifice for sampling said mixture ofthe first and second gases, said orifice being located between saidmixing means and said outlet means for the mixture, said devicefurthermore including means for sending the stream of the first gas intosaid chamber with a controlled flow rate and means for sending thestream of the second gas into said chamber with a controlled flow rate.

[0026] The advantages of the device according to the invention havealready been indicated in the foregoing description of the method, butit may be added that the device according to the invention is simple,reliable and uses only components that already exist commercially andare easily available.

[0027] The invention will now be described in detail in the followingdescription, given by way of non-limiting illustration, with referenceto the appended drawing in which:

[0028] the single figure is a schematic sectional view of a deviceaccording to the invention.

[0029] The single figure shows a device according to the invention,which comprises a mixing chamber (1), generally of approximately tubularshape, with a first end (2) and a second end (3).

[0030] The chamber, within which firstly a gas (4), such as air, flowsand then the mixture of the two gases flows, is for example made of ametal, such as stainless steel, and must be able to be heated to a highenough temperature, for example 200° C., in order to be able to desorbthe product(s) that have built up on the walls, if this is necessary.For this purpose, the chamber is generally provided with heating means(not shown) that may, for example, consist of a resistance heatingelement wound around the tubular chamber (1).

[0031] The first end (2) of the tubular chamber, or upper end in thepresent case, forms an inlet orifice for a stream (4) of the first gas.This first gas is, for example, air and is sent into the column via afan (5). It is quite obvious that the fan (5) may be replaced with anysuitable device for sending the stream of the first gas into saidchamber with a controlled flow rate. As an example of a device that canbe used for providing a controlled and steady stream of the first gas,such as air, in the chamber, mention may thus be made of a container,such as a bottle, of the compressed first gas, such as artificial air,that is connected to the inlet orifice via a flowmeter.

[0032] According to the invention, the flow rate of the first gas, suchas air, may be precisely adjusted and controlled at the outlet end, orlower end, of the chamber by a device such as an anemometer probe.

[0033] Into the stream of the first gas, blown by the fan (5) andflowing in the tubular chamber (1), is injected a stream of a second gasvia an inlet orifice (6) located on the side wall (7) of the chamber,downstream of, or below, the upper end (2) forming the inlet orifice forthe stream of the first gas.

[0034] The second gas is in fact generally formed by the vapor of aproduct initially in the liquid state.

[0035] This product is injected by means of a suitable device thatconstitutes the means according to the invention for sending orinjecting the stream of the second gas into the chamber with acontrolled flow rate.

[0036] This device is, in the single figure, shown in the form of asyringe (8) provided with a precision syringe plunger (9) of the typeused in the medical field, for example for dialysis.

[0037] However, in general, any system allowing controlled injectionwith a sufficiently low flow rate is suitable and can be fitted to thedevice of the invention. Thus, the means for sending the stream of thesecond gas into the chamber with a controlled flow rate could, forexample, be formed by an inkjet printer cartridge filled with thedesired product.

[0038] The product, initially in the liquid state, is generally anorganic compound selected from volatile organic compounds or a mixtureof the latter.

[0039] This product is generally a product that may be termed a“pollutant”, especially an atmospheric air pollutant, generally selectedfrom volatile organic compounds or a mixture of the latter. If theproduct injected is a mixture of several compounds, these are in knownfixed concentrations.

[0040] The liquid product, for example placed inside the syringe (8), isinjected with a controlled flow rate, of the order of 1 nl/min, forexample into a stream of the first gas, such as air, which also has aknown flow rate. The flow rate of the first gas, such as air, may forexample be of the order of m³/min and is very substantially greater thanthat of the injected product, in such a way that the saturated vaporpressure of the injected product is never reached and the productimmediately vaporizes on leaving the syringe, in contact with the gas,such as air. Because the flow rate of the stream of the first gas islargely in majority over that of the second gas, the expression“dilution of the second gas in the first gas” may be used.

[0041] The term “largely in majority” is generally understood to meanthat the flow rate of the first gas is from 10⁶ to 10¹² times greaterthan that of the second gas.

[0042] Provided downstream of the point of injection of the stream ofthe second gas are means (10) for mixing the gas streams in order toobtain a homogeneous mixture of these first and second gases, forexample air and pollutant, before the sampling outlet.

[0043] In the single figure, the mixing means (10) consist of one ormore static mixers, such as packing rings, for example the stainlesssteel RAFLUX® packing rings sold by Rauschert®, but other types ofmixers may be envisioned, for example one or more dynamic mixers, suchas one or more fans.

[0044] Provided downstream of the one or more mixers, for example theone or more static mixers, is a sampling orifice (11) located, in thesingle figure, on the side wall (7) of the chamber in order to sample,continuously or intermittently, a defined volume of the homogeneous gasmixture.

[0045] The orifice is provided with a stainless steel tube bent into aright angle toward the outlet of the device located at the end of thetubular chamber (3); the orifice has for example a diameter of about 3.2mm (⅛ of an inch). This homogeneous gas mixture contains an extremelyprecise concentration of the second gas in the first gas, for exampleone or more pollutants in the air.

[0046] The device according to the invention makes it possible toprepare mixtures with a wide concentration range. Thus, it is possibleto vary the concentrations of the second gas in the first gas, forexample one or more pollutants in the air, within a range from 1 ppmv(10⁻⁶) down to 1 pptv (10⁻¹²), and to do so always with perfectstability and very great reproducibility.

[0047] The remainder (12) of the stream of the homogeneous gas mixtureis discharged via a discharge orifice located at the end of the tubularchamber (3), in this case at the lower end.

[0048] Near this outlet orifice—in fact at the center of the section ofthe tubular chamber and slightly downstream of the lower end of thelatter—the flow rate of the gas stream which is in fact essentiallyformed by the first gas, such as air, is measured by suitable measuringmeans, such as an anemometer probe (13) connected to a display device(14) of the TESTO 435® type. By measuring the output flow rate,preferably continuously, it is possible to adjust precisely, and at anyinstant, the flow rate of the stream of the first gas entering thechamber.

[0049] The sampling orifice is connected, for example, to a gasconcentration meter (not shown) (i.e. one for measuring theconcentration of the second gas in the first gas, namely, for example,of the pollutant or pollutants in air), to a gas sensor or to apreconcentration instrument, because the gas mixture leaving thesampling orifice has an extremely precise concentration of the secondgas (for example of pollutant) and because it is possible to vary thisconcentration easily and precisely over a wide range. It is possible toimprove the calibration of this gas concentration meter very precisely,even if the concentrations involved are very low. Thus, the deviceaccording to the invention also makes it possible to check theperformance characteristics claimed by the manufacturers of theseinstruments.

[0050] The device according to the invention may be connected directly,or via a preconcentration system, to an instrument formed by thecoupling of a microchromatograph to a mass spectrometer (μGC/MS).

[0051] The invention will now be described in relation to the followingexamples, these being given as non-limiting illustrations.

EXAMPLE 1

[0052] This example employed a device very similar to that illustratedin the single figure, in which the instruments used were the following:

[0053] syringe plunger: supplied by Harvard Apparatus;

[0054] syringe (volume: 10 ml): supplied by Harvard Apparatus; and

[0055] fan: this was an extractor fan for ducting, supplied by S & P.The system was configured in such a way that the delivered flow ratecould be adjusted between 10 and 70 m³/h approximately; and

[0056] air speed indicator: TESTO 435® model, fitted with an anemometerprobe.

[0057] The first results were obtained using, as pollutant, n-butanol(density: 0.81 g/cm³; molar mass: 74.12 g/mol).

[0058] The injection flow rates obtained with the equipment describedabove were of the order of a few μl/min.

[0059] For example, with an injection flow rate set at 11 μ/min, theamount of n-butanol injected was therefore:${11\quad {{µ1}/\min} \times \frac{1\quad {ml}}{1000\quad µ\quad 1} \times 0.81\quad g\text{/}{ml}} = {9\quad {mg}\text{/}{\min.}}$

[0060] This injected mass represented a volume of vapor at 20° C. and atatmosphere pressure of:$\frac{24000\quad {ml}\text{/}{mol} \times 9 \times 10^{3}\quad g\text{/}\min}{74.12\quad g\text{/}{mol}} = {2885\quad µ\quad 1\text{/}\min}$

[0061] i.e. 1.7×10⁻⁴ m³/h.

[0062] The air flow rate was constant and set at 19 m³/h; thetheoretical n-butanol concentration at the outlet was therefore:${\frac{1.7 \times 10^{- 4}{m^{3}/h}}{19\quad {m^{3}/h}} \times 10^{6}} = {9\quad p\quad p\quad m\quad {V.}}$

[0063] If the flow rate of the injected product were to be increased to20 μl/min, with the air flow rate remaining unchanged, the concentrationwould then be 17 ppmv.

[0064] The change in the n-butanol concentration was monitored in line,using μGC/MS coupling and without passing via a preliminary accumulationsystem. The pollutant concentration was a function of the area of thechromatograph peak, obtained by the microcatharometer detector making upthe coupling. This instrument made it possible to carry out an analysisapproximately every 2 minutes, and various curves of variation were thusable to be plotted in the case of n-butanol.

[0065] This example demonstrates the advantages that are in generalobtained with the device and the method of the invention compared withthe devices and methods of the prior art. These advantages are inparticular the possibility of varying the concentration of the pollutantstudied, by changing the injection flow rate, the new mixture thenbecoming steady in barely a few minutes (for example 5 minutes).

1. A method of mixing, in dynamic mode, a second gas in a first gas, inwhich a stream of the second gas is introduced into a stream of thefirst gas, said streams of gases having controlled flow rates, and saidstreams of the first and second gases are mixed so as to obtain ahomogeneous mixture of the two gases that has a defined concentration ofthe second gas.
 2. The method as claimed in claim 1, in which said firstgas is chosen from air, nitrogen, argon, helium and mixtures thereof. 3.The method as claimed in claim 1, in which said second gas results fromthe vaporization of a liquid compound, preferably selected from vaporsof liquid organic compounds and mixtures thereof.
 4. The method asclaimed in claim 1, in which said second gas consists of a compoundselected from compounds polluting the atmospheric air and mixturesthereof.
 5. The method as claimed in claim 4, in which said pollutingcompounds are selected from volatile organic compounds.
 6. The method asclaimed in claim 1, in which the flow rate of the first gas is largelyin majority over the flow rate of the second gas.
 7. The method asclaimed in claim 6, in which the flow rate of the first gas is from 10⁶to 10¹² times greater than that of the second gas.
 8. The method asclaimed in claim 1, in which a fraction of the homogeneous mixture ofthe two gases is sampled and sent into a meter and/or detector and/orconcentration meter.
 9. A device for mixing, in dynamic mode, a secondgas in a first gas, said device comprising a substantially tubularmixing chamber (1), a first inlet orifice for a stream (4) of the firstgas at one end (2) of said chamber, a second inlet orifice (6) for astream of the second gas, located downstream of said first orifice inthe direction of flow of the stream of the first gas, means (10) forhomogeneously mixing said streams of the first and second gases, saidmeans being located downstream of said second inlet orifice, an outletorifice (3) for the mixture, located downstream of said mixing means(10), at the other end of said chamber, and an orifice (11) for samplingsaid mixture of the first and second gases, said orifice being locatedbetween said mixing means (10) and said outlet means (3) for themixture, said device furthermore including means (5, 8, 9) for sendingthe stream of the first gas into said chamber with a controlled flowrate and for sending the stream of the second gas into said chamber witha controlled flow rate.
 10. The device as claimed in claim 9, in whichsaid means for mixing said streams of the first and second gases areformed by one or more static mixers.
 11. The device as claimed in claim9, in which said means for mixing said streams of the first and secondgases are formed by one or more dynamic mixers, such as one or morefans.
 12. The device as claimed in claim 9, in which said means forsending the stream of the first gas into said chamber with a controlledflow rate are formed by a container for the compressed first gas, andconnected to said first inlet orifice via a flow meter, or by a fan. 13.The device as claimed in claim 9, in which said means for sending thestream of the second gas into said chamber with a controlled flow rateare formed by a syringe fitted with a precision syringe plunger.
 14. Thedevice as claimed in claim 9, in which said means for sending the streamof the second gas into said chamber with a controlled flow rate areformed by an inkjet printer cartridge.
 15. The device as claimed inclaim 9, in which the mixing chamber is provided with heating means. 16.The device as claimed in claim 9, in which the sampling orifice isconnected to a gas concentration meter, to a preconcentration instrumentor to a gas sensor.
 17. The device as claimed in claim 16, in which thesampling orifice is connected directly, or via a preconcentrationsystem, to an instrument formed by the coupling of a microchromatographand a mass spectrometer.